A spectrometer disperses light into different colors, as is well known. In order to achieve a high degree of separation of light of different colors, the optical opening in the dispersing direction typically is made as small as possible to prevent overlapping of colors. Thus, the optical opening along say the horizontal axis of the object plane is small. On the other hand, in order to obtain sufficient light input to the spectrometer, a large optical opening along the vertical (nondispersive) axis is employed. The required optical opening thus is rectangular in shape and is commonly called a slit. Such a slit is characteristic of spectrometers. Typically, the vertical dimension of the slit is 1,000-10,000 times larger than the horizontal dimension.
Unfortunately, the use of a slit in a spectrometer is attended by several problems. First, there is a tradeoff of small slit-width to achieve resolution, against larger slit-width to gain light. Second, slits and the attendant mechanisms for opening and closing them are expensive. The expense arises because any variation in width must be realized equally at all heights, i.e., the width must be the same everywhere. If this is not achieved, resolution is reduced and aberrations are introduced. Normally, two precisely machined, straight slit-jaws are moved to and from one another in a precisely parallel, reproducible, and measurable fashion. This involves precision mechanisms that are costly and require maintenance and replacement.
A further problem arises just from the geometry of the slit. The slit is hardly a point source. Thus, additional optical aberrations are introduced by its use. These aberrations cannot all be avoided. Nevertheless, attempts to minimize then must be and are made. Some of these attempts cause loss of light, and are usually very expensive.
A still further problem results from the operative nature of the slit. A slit acts to aperture or block light. Consequently, a shape mismatch between a source and an associated slit occurs. Even with a narrow laser beam, a slit must block out light in the dispensing direction or the aperturing function fails, i.e., the beam apertures itself. If the beam width is smaller than the slit-width, on the other hand, the slit fails to operate as a slit, and beam vibrations can produce noise. When the beam width is greater than the slit-width, loss of light occurs. It is clear then, that the correct use of a slit must involve loss of light.
For high-resolution operation, a small slit width must be employed. This results in a reduced light level. Also, diffraction patterns are formed and resolution is limited as a consequence.